1. Field of the Invention
The present invention relates to a fuel cell in which hydrogen gas is reacted with oxygen in the atmosphere to generate power, and more particularly to a fuel cell in which moisture produced by the power generation is effectively utilized to reduce nonuniformity of wetness (i.e., degree of wetness) in the planar direction of an electrolyte membrane.
2. Description of the Related Art
A fuel cell using hydrogen gas as a fuel is significantly larger in the amount of energy supplied per unit volume compared with a conventional secondary battery and the whole structure thereof can be repetitively utilized by filling with the fuel. The fuel cell can be configured without adopting a movable mechanism and therefore makes less noise and generates no carbon dioxide during the power generation. For this reason, the fuel cell is expected to be applied as a light-weight, small-sized, long-life, pollution-free power supply in small electric devices such as mobile phones, notebook personal computers, personal digital assistants, electric shavers, and cordless cleaners.
U.S. Pat. No. 5,514,486 discloses a fuel cell in which a fuel supply layer and an oxygen supply layer are disposed on both sides of a polymer electrolyte membrane. In the fuel cell, a hole for supplying hydrogen gas serving also as a through hole for a connecting bolt is provided at the center. A fuel supply layer, a polymer electrolyte membrane layer, an oxygen supply layer, and a separator each formed in a circular shape are stacked in a given order. Further, hydrogen gas is reacted with oxygen in the atmosphere without using a pressure pump and blower, and the polymer electrolyte membrane is humidified by moisture as a reaction product which diffuses and moves in the oxygen supply layer.
Japanese Patent Application Laid-Open No. 2000-58100 discloses a fuel cell in which power generation cells are staked without using separators. A membrane electrode assembly is a functional material in which electrode layers are integrally formed on the front and rear surfaces of a polymer electrolyte membrane. Such membrane electrode assemblies are disposed on the front and rear surfaces of an oxygen supply layer whose side surfaces are open to the atmosphere, and a pair of fuel supply layers are disposed on the front and rear sides of the oxygen supply layer through the membrane electrode assembly.
In J. Electrochem. Soc., Vol. 144, No. 8, P 2767-2772, August 1997, The Electrochemical Society, Inc. “Operating Proton Exchange Membrane Fuel Cells Without External Humidification of the Reactant Gases”, Felix N. Büchi and Supramaniam Srinivasan, a fuel cell is described in which the direction of hydrogen gas flow in a fuel supply layer is set to be opposite to the direction of oxygen flow in an oxygen supply layer on the opposite surfaces of a membrane electrode assembly. In this fuel cell, moisture produced on the oxygen supply layer side of the membrane electrode assembly is carried by air flow and conveyed to a downstream region to relatively increase the wetness of the membrane electrode assembly in the downstream region (see FIG. 7). In the downstream region where the wetness has been increased, water molecules permeate the fuel supply layer through the membrane electrode assembly and humidify hydrogen gas in an upstream region.
A polymer electrolyte membrane conveys hydrogen ions in a state in which a substantial amount of water molecules is held in the molecular structure thereof, so that the power generation efficiency is lowered if the membrane loses wetness due to evaporation or drying to decrease power density of a power generation surface. In particular, at the time of activation after a long period of suspension, the polymer electrolyte membrane is in a semi-dry state and is low in power generation efficiency. Therefore, when the heat generation of the polymer electrolyte membrane is increased with the start of the power generation, the humidification by the formation of water cannot follow the drying partly. This may cause delay in the rise of an output voltage of the fuel cell to be a disadvantage for the activation of an electric device having the fuel cell mounted thereon.
Accordingly, in the fuel cell described in U.S. Pat. No. 5,514,486, it is necessary to supply water to the oxygen supply layer prior to the activation or to incorporate a humidifier into a hydrogen gas supplying system, thereby ensuring the wetness of the polymer electrolyte membrane.
In the fuel cell described in J. Electrochem. Soc., Vol. 144, No. 8, P 2767-2772, August 1997, The Electrochemical Society, Inc. “Operating Proton Exchange Membrane Fuel Cells Without External Humidification of the Reactant Gases”, Felix N. Buchi and Supramaniam Srinivasan, since an air flow in the oxygen supply layer is utilized to cause the produced water to extend over the entire surface of the polymer electrolyte membrane, neither supplying water to the oxygen supply layer nor incorporating a humidifier for hydrogen gas is unnecessary. However, since the circulation of produced water depends on an air flow in the oxygen supply layer, there is a possibility that a deficiency of the air flow may result in insufficient distribution of moisture, whereby the effect cannot be exhibited satisfactorily.
In the fuel cell described in Japanese Patent Application Laid-Open No. 2000-58100, the supply of oxygen to the membrane electrode assembly entirely depends on the natural diffusion of oxygen in the atmosphere and there is no steady flow. Therefore, water produced at a portion of the membrane electrode assembly where power generation is actively performed permeates the fuel supply layer and is conveyed to the downstream region of hydrogen gas flow, so that the wetness of the polymer electrolyte membrane in the upstream region cannot easily be increased.
For this reason, it takes a long period of time to wet the entire surface of the polymer electrolyte membrane satisfactorily to attain a high power generation density.